Mechanics

FUEL PUMP

1996 Mercedes Benz 500sl

Engine Performance problem
1996 Mercedes Benz 500sl V8 Two Wheel Drive Automatic 103000 miles

about 2 months ago I started to notice a intermitent hesitation at times when I am driving down the road or starting from a stand still. It does NOT happen all the time. I can drive for 20 miles and have no problem or drive 3 miles and have the problem. It does not backfire but feels like it is not getting gas. I replaced all the plugs and plug seals but still having problems. It happens more when I stomp down on the gas but again not all the time. My father thought it might be the mass air flow but I am seeing no lights on the panel telling me of any problems. When I speak with a mercedes mechanic about the fuel filter, they said they almost never have a problem in that area. How do I tell if it is the fuel pump?

thanks
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Dcrockett46
February 13, 2009.



There are a few things to consider with this issue. First, your dad may be right. A Mass Air Flow sensor can definately cause this and will not always set a check engine light. If you live in a particularly dusty place this could be the case. As for the mechanics comment on fuel filters, MB recommends a new fuel filter every 60k miles. Not only could this be your cause, this would be the cheapest and easiest place to start. And more than likely you need one anyways. Also there is a known issue with fuel pressure regulators leaking internally on this vehicle. The faulty regulator will cause problems similar to yours. As for the fuel pump. I have seen very few fuel pumps replaced on 129 chassis of SL's. It is possible that this could be causing your issue however this is very expensive to replace and I would be very hesitant to go after this first. Many times when a pump goes bad it begins to hum and make excessive noise. You can ohm out the pump with a multi meter to check for high resistence as well. I however do not have the figures for spec on this.

Gfimotorcars
Feb 13, 2009.
You also mentioned the fuel pressure regulator might be leaking? How do I tell if it leaking and how do I replace it? It is hard?

Thanks

Tiny
Dcrockett46
Feb 13, 2009.
This is a specific faulty part so it will be necessary to check the part # to see if it is part of the faulty batch originally shipped. The faulty part # is:

000-078-15-89 (Brumme production code 06.95 and 12.95)

The fuel pressure regulator is located as illustrated below. Diaphragm Pressure Regulator #40


http://www.2carpros.com/forum/automotive_pictures/294900_129_diaphragm_pressure_regulator_1.jpg



It will be on the driverside front of the motor. Along the fuel lines. You should see the part number on the regulator

Gfimotorcars
Feb 13, 2009.